Abstract. The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO 2 ) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO 2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO 2 -concentration, reactive uptake coefficients γ rxn for the NO 2 →HONO conversion on the aerosol between γ rxn <10 −7 (in the dark) and γ rxn =6×10 −6 were observed. The observed uptake coefficients decreased with increasing NO 2 -concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (<20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO 2 →HONO conversion are too low to explain the HONOformation rates observed near the ground in rural and urban environments by the conversion of NO 2 →HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that the processes leading to HONO formation on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.

Mineral dust contains material such as TiO2 that is well known to have photocatalytic activity. In this laboratory study, mixed TiO2‐SiO2, Saharan dust and Arizona Test Dust were exposed to NO2 in a coated wall flow tube reactor. While uptake in the dark was negligible, photoenhanced uptake of NO2 was observed on all samples. For the mixed TiO2‐SiO2, the uptake coefficients increased with increasing TiO2 mass fraction, with BET uptake coefficients ranging from 0.12 to 1.9 × 10−6. HONO was observed from all samples, with varying yields, e.g., 80% for Saharan dust. Three‐dimensional modeling indicates that photochemistry of dust may reduce the NO2 level up to 37% and ozone up to 5% during a dust event in the free troposphere.

[1] The nitrate formation on dust particles is considered as a sink for atmospheric NO y (such as HNO 3 ). However mineral dust is shown here to be an effective photocatalyst for transformation of nitrate anions into NO and NO 2 , without involving its photolysis. The photodecomposition of NO 3 À at the surface of synthetic mineral dust samples of SiO 2 , TiO 2 , mixed TiO 2 -SiO 2 and authentic sand doped with 6% NO 3 À was studied by means of a flow-tube at 298 K with UVillumination in the 340 -420 nm range at relative humidities between 5 and 80%. Both NO and NO 2 are observed during irradiation of films composed of either mixed TiO 2 -SiO 2 , pure TiO 2 and authentic minerals from the Sahara. The relative humidity strongly affects the concentration of NO x released into the gas phase. The photoinduced nitrate conversion into NO x is discussed as being a potential renoxification process of the atmosphere.

The ozone decomposition onto mineral surfaces prepared with traces of solid TiO2 in a matrix of SiO2 in order to mimic mineral dust particles has been investigated using a coated-wall flow-tube system at room temperature and atmospheric pressure. The ozone uptake coefficients were measured both under dark conditions and irradiation using near UV-light. While uptake in the dark was negligible, a large photoenhanced ozone uptake was observed. For TiO2/SiO2 mixtures under irradiation, the uptake coefficients increased with increasing TiO2 mass fraction (from 1 to 3 wt %), and the corresponding uptake coefficient based on the geometric surfaces ranged from 3 x 10(-6) to 3 x 10(-5). The uptake kinetics was also observed to increase with decreasing ozone concentration between 290 and 50 ppbv. Relative humidity influenced the ozone uptake on the film, and a reduced ozone loss was observed for relative humidity above 30%. The experimental results suggest that under atmospherically relevant conditions the photochemistry of dust can represent an important sink of ozone inside the dust plume.

The heterogeneous reaction of NO(2) on Saharan sand collected from different locations has been studied at 298 K and 25% relative humidity using a horizontal coated-wall flow tube. The sand samples originated from Mauritania, Algeria, Morocco and Tunisia and were taken as simplified proxies for mineral dust. While the uptake in the dark was always very small, a photo-enhanced uptake of NO(2) was observed on all four samples showing that natural minerals do have a photochemical activity. The uptake coefficient gamma(BET) was measured for all sands. In the dark, the gamma(BET) values are (1.60 +/- 0.24) x 10(-8), (0.43 +/- 0.06) x 10(-8), (0.94 +/- 0.14) x 10(-8) and (0.59 +/- 0.09) x 10(-8) for the samples from Mauritania, Algeria, Morocco and Tunisia, respectively. Under realistic atmospheric conditions, the observed photo-enhancement leads to uptake coefficients of (1.46 +/- 0.21) x 10(-7), (0.35 + 0.05) x 10(-7), (1.30 +/- 0.19) x 10(-7) and (0.89 +/- 0.13) x 10(-7), respectively, i.e. an enhancement factor ranging from 8 to 15. This study shows that the photochemistry of natural minerals will impact significantly on the heterogeneous chemistry of NO(2).

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Abstract. The interactions of aerosols consisting of humic acids with gaseous nitrogen dioxide (NO2) were investigated under different light conditions in aerosol flow tube experiments at ambient pressure and temperature. The results show that NO2 is converted on the humic acid aerosol into nitrous acid (HONO), which is released from the aerosol and can be detected in the gas phase at the reactor exit. The formation of HONO on the humic acid aerosol is strongly activated by light: In the dark, the HONO-formation was below the detection limit, but it was increasing with the intensity of the irradiation with visible light. Under simulated atmospheric conditions with respect to the actinic flux, relative humidity and NO2-concentration, reactive uptake coefficients γrxn for the NO2→HONO conversion on the aerosol between γrxn <10−7 (in the dark) and γrxn = 6×10−6 were observed. The observed uptake coefficients decreased with increasing NO2-concentration in the range from 2.7 to 280 ppb and were dependent on the relative humidity (RH) with slightly reduced values at low humidity (<20% RH) and high humidity (>60% RH). The measured uptake coefficients for the NO2→HONO conversion are too low to explain the HONO-formation rates observed near the ground in rural and urban environments by the conversion of NO2→HONO on organic aerosol surfaces, even if one would assume that all aerosols consist of humic acid only. It is concluded that humic materials present on the Earth surface will have a much larger impact on the HONO-formation in the lowermost layer of the troposphere than humic materials potentially occurring in airborne particles.

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